Although tumor necrosis factor (TNF) exhibits significant antitumor activity toward some cell cancer lines in vitro, it has not been notably successful in the clinic. This is partly due to the negative side effects of the required levels of TNF in vivo and, perhaps more importantly, many tumors are, or become, resistant to TNF treatment.
It is believed that TNF is internalized through interaction with a TNF receptor, and a number of mechanisms have been postulated for the action of the internalized TNF. Agents that block TNF internalization or lysosomal enzyme function protect cells from TNF-mediated killing (Watanabe, N., et al., Immunopharm Immunotox (1988) 10:109-196; Aggarwal, B. B., J Biol Chem (1986) 261:13652-13656; Liddel, J. D., et al., Cancer Res (1989) 49:2717-2718). It has been shown that TNF-sensitive murine L-929 cells release a degradation product into the media which disrupts lysosomes; TNF-resistant L-929 internalize TNF but do not produce a degradation product (Ohsawa, F., et al., J Biochem (1988) 103:730-734).
In a previous study, the inventors herein reported that MCF-7 cells that were sensitive to TNF internalized TNF and degraded it to a 15 kd species and appeared to recycle the receptor. On the other hand, MCF-7 cells that were resistant to TNF produced multiple lower molecular weight TNF products and showed decreased binding over time consistent with lysosomal degradation of the receptor TNF complex (Fruehauf, J. P., et al., J Immunotherapy (1991) 10:165-173, incorporated herein by reference). This pattern was also shown in prostate cell lines (Freuhauf, J. P., et al., J Nat Cancer Inst (1990) 82:12-16-1207).
It has now been found that a fraction of conditioned media from TNF-treated TNF-sensitive cells is cytotoxic both to TNF-sensitive and TNF-resistant tumor cells.
Previous reports have suggested that TNF resistance may be related to growth factor receptor expression, including those of Sugarman, B. J., et al., Cancer Res (1987) 47:780-786; Lichtenstein, A., et al., Cancer Res (1990) 7364-7370; Hudziak, R. M., et al., Mol Cell Biol (1989) 9:1165-1162. It has now also been shown that EGF receptor expression correlates with TNF resistance.